1. Field of the Invention
The present invention relates to a semiconductor device and manufacturing method thereof and, more specifically, to a semiconductor device in which electrical insulation of a transistor is ensured, as well as to the manufacturing method thereof.
2. Description of the Background Art
A method of manufacturing a semiconductor device including an MOS transistor, as an example of a conventional semiconductor device, will be described with reference to the figures, First, referring to FIG. 23, a trench element isolating film 102 for forming an element forming region is formed on a silicon substrate 101. Thereafter, a thermal oxide film (not shown) is formed on a surface of silicon substrate 101 by thermal oxidation.
On the thermal oxidation film, a polycrystalline silicon film and a silicon oxide film (both not shown) are formed by the CVD method. On the silicon oxide film, a prescribed photoresist pattern (not shown) is formed. Using the photoresist pattern as a mask, the silicon oxide film, the polycrystalline silicon film and the thermal oxide film are anisotropically etched, so that a gate oxide film 106, a polysilicon gate 107 and a silicon oxide film 108 are formed.
Thereafter, as shown in FIG. 24, a silicon oxide film 109 is formed by the CVD method on silicon substrate 101 to cover polysilicon gate 107 and the like. Thereafter, as shown in FIG. 25, silicon oxide film 109 is anisotropically etched, so as to form sidewall oxide films 109a on the side surfaces of polysilicon gate 107.
Thereafter, as shown in FIG. 26, an impurity producing a prescribed conductivity type is introduced by ion implantation into the surface of silicon substrate 101. Further, by heat treatment, a pair of source and drain regions 111a and 111b are formed.
Thereafter, referring to FIG. 27, silicon films 112a and 112b are formed selectively only on the surfaces of source and drain regions 111a and 111b by selective epitaxial growth. Thereafter, as shown in FIG. 28, an impurity of the same conductivity type as source and drain regions 111a and 111b are introduced by ion implantation into selective silicon films 112a and 112b, and by heat treatment, the selective silicon films 112a and 112b are adapted to have lower resistance, whereby low resistance selective silicon films 113a and 113b are formed, respectively.
In this manner, an MOS transistor T including a polysilicon gate 107, a pair of source and drain regions 111a and 111b and low resistance selective silicon films 113a and 113b are formed.
Thereafter, referring to FIG. 29, an interlayer silicon oxide film 114 is formed by the CVD method on silicon substrate 101 to cover the MOS transistor T. On the interlayer silicon oxide film 114, a prescribed photoresist pattern (not shown) is formed. Using the photoresist pattern as a mask, the interlayer silicon oxide film 114 is anisotropically etched, whereby contact holes 121a and 121b exposing surfaces of low resistance selective silicon films 113a and 113b are formed, respectively.
Thereafter, a prescribed metal film is formed by sputtering, for example, and a prescribed heat treatment is performed. Thus, a titanium silicide film 115, a titanium nitride film 116 and a tungsten film 117 are formed in contact holes 121a and 121b. Thereafter, on the interlayer silicon oxide film 114, a prescribed metal interconnection (not shown) electrically connected to tungsten film 117 is formed. An interlayer insulating film and a passivation film (both not shown), for example, are further formed on interlayer silicon oxide film 114, to cover the metal interconnection. By prescribed photolithography and etching, interconnection pads and the like are formed.
The conventional semiconductor device is thus completed through the above described steps.
As described above, in the conventional semiconductor device, the sidewall oxide film 109a of silicon oxide film has been applied as a sidewall insulating film. Here, in the step of FIG. 27, when selective silicon films 112a and 112b are formed, selectivity of silicon epitaxial growth is established on the source and drain regions 111a and 111b (silicon substrate) and on the sidewall oxide film 109a (silicon oxide film) adjacent thereto, and therefore it is possible to form selective silicon films 112a and 112b selectively only on the source and drain regions 111a and 111b. 
There has been a demand to increase degree of integration of the semiconductor devices for miniaturization. In order to meet such a demand, use of a silicon nitride film in place of the conventional silicon oxide film as the sidewall insulating film is expected. This is to ensure registration margin for photolithography when contact holes 121a and 121b are to be formed in interlayer silicon oxide film 114.
Use of the silicon nitride film as the sidewall insulating film may lead to the following problem. The silicon nitride film has lower selectivity with respect to the silicon substrate at the time of silicon epitaxial growth, as compared with the silicon oxide film. Therefore, in the process of epitaxial growth of the selective silicon film having the prescribed film thickness, silicon island 118 would be formed on the surface of sidewall insulating film 130 of silicon nitride film, as shown in FIG. 30.
If such silicon islands 118 are formed, the selective silicon film 112b would be electrically connected to other selective silicon film through the silicon island 118, possibly causing electric short-circuit of source and drain region 111b with other source and drain regions. As a result, the semiconductor device cannot perform a desired operation.
The present invention was made to solve the above described problem. One object of the present invention is to provide a semiconductor device capable of ensuring electrical insulation and another object is to provide a method of manufacturing such a semiconductor device.
According to an aspect, the present invention provides a semiconductor device including a semiconductor substrate having a main surface, first and second conductive layers, a silicon nitride film and a protective layer. The first and second conductive layers are formed spaced from each other on the main surface of the semiconductor substrate. The silicon nitride film is formed on the main surface of the semiconductor substrate traversing between the first and second conductive layers. The protective layer is formed on the surface of the silicon nitride film and at least until the first and second conductive layers are formed to a prescribed film thickness, prevents deposition of the material of the first and second conductive layers on the surface of the silicon nitride film.
In this structure, deposition of the material of the first and second conductive layers on the silicon nitride film can be prevented by the protective film until the first and second conductive layers are formed to a prescribed film thickness. As a result, electrical short-circuit between the first and second conductive layers through the material can be prevented.
Especially when the semiconductor substrate includes a silicon substrate and the first and second conductive layers include silicon epitaxial growth layer, the protective layer should preferably be a layer preventing the material of the epitaxial growth layer. Here, it is possible to prevent deposition of silicon pieces on the surfaces of the silicon nitride film. Preferably, the semiconductor device includes a pair of impurity regions of a prescribed conductivity type formed spaced from each other on the main surface of the semiconductor substrate, an electrode portion formed on the main surface of the semiconductor substrate sandwiched between the pair of impurity regions, and sidewall insulating films formed on the side surfaces of the electrode portion, in which the first and second conductive layers are formed on the surfaces of one end and the other of the pair of impurity regions, and the sidewall insulating film includes a silicon nitride film and a protective layer.
Here, in the transistor including a pair of impurity regions and the electrode portion, it is possible to prevent electrical short circuit between one impurity region with the other impurity region or with an impurity region of another transistor, whereby operation of the transistor can be made stable. Further, as the sidewall insulating film has a silicon nitride film, it is possible to ensure registration margin for photolithography when a prescribed contact hole is to be formed near the electrode portion, as will be described later.
In order to prevent deposition of the material to be the first and second conductive layers, it is preferred that the protective layer includes at least an element selected from the group consisting of oxygen, hydrogen and halogen.
Preferably, the prescribed thickness of the first and second conductive layers is at least 50 nm. With this thickness, it is possible to form the first and second conductive layers with high precision.
According to a second aspect, the present invention provides the method of manufacturing a semiconductor device including the following steps. A silicon nitride film is formed on a prescribed region on a main surface of a semiconductor substrate. First and second conductive layers are formed by epitaxial growth on one and the other portions on the main surface of a semiconductor substrate sandwiching a prescribed region. Between the step of forming the silicon nitride film and the step of forming the first and second conductive layers, a prescribed process is performed on the silicon nitride film, so that a protective layer is formed on a surface of the silicon nitride film, the protective layer preventing deposition of the material of the first and second conductive layers on the surface of the silicon nitride film, until the first and second conductive layers reach a prescribed film thickness.
By this manufacturing method, while the first and second conductive films of the prescribed film thickness are formed by epitaxial growth, a protective layer is formed on the silicon nitride film, so that it becomes possible to prevent deposition of the material of the first and second conductive layers on the surface of the silicon nitride film. As a result, electrical short-circuit between the first and second conductive layers through such a material can be prevented.
Preferably, the manufacturing method includes the steps of forming a pair of impurity regions of a prescribed conductivity type electrically connected to the first and second conductive layers, respectively, forming an electrode portion on a prescribed region of the semiconductor substrate, and forming a sidewall insulating film including a silicon nitride film and a protective layer on a side surface of the electrode portion.
Here, a transistor including the electrode portion and the pair of impurity regions is formed, and in the transistor, it is possible to prevent electrical short-circuit of one impurity region with the other impurity region or with an impurity region of another transistor. As a result, a semiconductor device with the transistor operation made stable can be obtained.
The step of forming the sidewall insulating film specifically includes the following two steps. One is the step of forming a protective layer by a prescribed processing on the silicon nitride film form to cover the electrode portion, and anisotropically etching the silicon nitride film and the protective layer. Another is the step of anisotropically etching the silicon nitride film formed on the semiconductor substrate to cover the electrode portion, so as to leave the silicon nitride film on the side surface of the electrode portion, followed by the prescribed processing.
The prescribed processing mentioned above specifically includes the following process. First, heat treatment is performed in an oxygen gas atmosphere. Water vapor may be added to the oxygen gas atmosphere. Hydrogen may be added to the oxygen gas atmosphere. Ozone may be added to the oxygen gas atmosphere. Aside from the heat treatment in the oxygen atmosphere, the heat treatment may be performed in a hydrogen gas atmosphere. The heat treatment may be performed in a halogen gas atmosphere. The heat treatment may be performed in ozone atmosphere. Further, heat treatment may be performed in an activated oxygen atmosphere. Further, heat treatment may be performed in an activated hydrogen gas atmosphere. At least one selected from the group consisting of oxygen, hydrogen and halogen may be introduced by ion implantation. By such a process, it is possible to form a desired protective layer on the surface and in the vicinity of the silicon nitride film.
Preferably, the prescribed thickness of the first and second conductive layers is at least 50 nm. With this thickness, it is possible to form the first and second conductive layers with high precision.